Re: [PATCH mm v3] memcg: enable memory accounting in __alloc_pages_bulk

On Wed 13-10-21 09:41:15, Shakeel Butt wrote:

On Tue, Oct 12, 2021 at 11:24 AM Michal Hocko [off-list ref] wrote:

quoted

On Tue 12-10-21 09:08:38, Shakeel Butt wrote:

quoted

On Tue, Oct 12, 2021 at 8:36 AM Michal Hocko [off-list ref] wrote:

quoted

On Tue 12-10-21 17:58:21, Vasily Averin wrote:

quoted

Enable memory accounting for bulk page allocator.

ENOCHANGELOG

And I have to say I am not very happy about the solution. It adds a very
tricky code where it splits different charging steps apart.

Would it be just too inefficient to charge page-by-page once all pages
are already taken away from the pcp lists? This bulk should be small so
this shouldn't really cause massive problems. I mean something like

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b37435c274cf..8bcd69195ef5 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c

@@ -5308,6 +5308,10 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,

        local_unlock_irqrestore(&pagesets.lock, flags);

+       if (memcg_kmem_enabled() && (gfp & __GFP_ACCOUNT)) {
+               /* charge pages here */
+       }

It is not that simple because __alloc_pages_bulk only allocate pages
for empty slots in the page_array provided by the caller.

The failure handling for post charging would be more complicated.

If this is really that complicated (I haven't tried) then it would be
much more simple to completely skip the bulk allocator for __GFP_ACCOUNT
rather than add a tricky code. The bulk allocator is meant to be used
for ultra hot paths and memcg charging along with the reclaim doesn't
really fit into that model anyway. Or are there any actual users who
really need bulk allocator optimization and also need memcg accounting?

Bulk allocator is being used for vmalloc and we have several
kvmalloc() with __GFP_ACCOUNT allocations.

Do we really need to use bulk allocator for these allocations?
Bulk allocator is an bypass of the page allocator for performance reason
and I can see why that can be useful but considering that the charging
path can imply some heavy lifting is all the code churn to make bulk
allocator memcg aware really worth it? Why cannot we simply skip over
bulk allocator for __GFP_ACCOUNT. That would be a trivial fix.
-- 
Michal Hocko
SUSE Labs